It isn't unreasonable, but I think quite a few people can't read any Japanese at all, so I'd guess that there's confusion sometimes whether what's written is random noises or actually something pertinent.
Personally, I find the untranslated tag to be useless in situations such as this one. Although, I guess my opinion is biased given that I can read hiragana and katakana. *shrugs* I would still drop a quick "translation" (such as, "random yuing") whenever you spot something like this before deleting the tag though.
Sure, you can leave off the untranslated tag for something like this. Here, it's only really a few effects around the side and they're not really prominent. If they're saying an actual sentence (even if it's something that we can recognise e.g. 'take it easy'), you should keep it in, or if there's a lot of them and they're quite prominent in the picture.
If you want a simpler way of filtering everything, you can use the tags in combination: untranslated -translation_available